The objective is to gain a detailed insight into the molecular events that are triggered by lymphocyte-recognition of foreign antigens, which lead to the generation of the proper immune responses. The results of these studies will enhance our understanding of the means by which cells of the immune system can deal with viral infections, some virally-induced malignancies organ transplantations and invading microorganisms. The specific aims of the proposed research are: 1. To study the role of accessory molecules like L3T4, Lyt-2, and LFA-1 in the interactions of T cells with their targets. 2. To identify membrane proteins that, following antigen-recognition, are involved in transmitting transmembrane signals. 3. To prepare new monoclonal antibodies in order to identify new cytoskeletal proteins and membrane proteins which become segregated into the effector-target contact area. 4. To find if T cell and B cell activation involves tyrosine kinase activity, and if so, to identify the phosphorylated proteins. 5. To determine if B cell activation by antigen induces intracellular events like the ones induced during T cell activation. 6. To localize B cell receptors during their interaction with specific T-helper cells.